"As NATO Allies, we have Article 5 duties to our collective defense. That is a commitment that is unbreakable, it is unwavering, it is eternal. Estonia will never stand alone," Obama told a news conference in Tallinn.

The US president stated that Air Force exercises from Amari air base in Estonia are planned.

"Today I can announce that this initiative will include additional Air Force units, an aircraft for training exercises here in the Nordic-Baltic region. And we agree with our Estonian Allies that an ideal location to host and support these exercises would be Amari air base here in Estonia," said Obama.

He pledged to discuss the initiative with the presidents of Lithuania and Latvia in Tallinn later on Wednesday.

In response to the Russian actions in Ukraine, the United States earlier in 2014 decided to step up the Baltic air-policing mission and send a company of troops to each of the three Baltic states and Poland.

Speaking at a joint news conference with Estonia's President Toomas Hendrik Ilves, Obama urged NATO member-states to follow the Estonian example and secure 2 percent of their gross domestic product (GDP) for defense.

In his words, Europe was for some time calm about the commitments to NATO, however, the Ukrainian crisis was a wake-up call.

"For a certain period of time, there was a complacency here in Europe about the demands that were required to make sure that NATO was able to function effectively," said the US president.